Translucency for shell material using the Principled Shader
Ok, the trick here is to have a mask control the vertical falloff of your shader. 

I dont have much time to work on this, but this should get you going in the right direction. I will also tell you what I would change. 






Getting the top translucent part : Your 1st shader should have the transmission all the way to 1 (for glass), then put a slight roughness on it. 
Have that mix in with another shader for the base of the shell, (we will get back to this)
Have those two shaders mix with the "Gradient Top" node* (You will have to create this, I attached a pic how to do it.)
The base of the shell will have 2 Principled SSS shaders, I believe I put the SSS radius to .1 for all of them, the higher the number the more transparent it becomes. 
I found a wood texture that I used for the grooves that I plugged into the principled shader. (I should of plugged it into both, I messed up. ) 
I used a noise texture to have the yellow and orange broken up a little. 
Then for the bands I just used a wave texture in the displacement socket. I would rather model this instead and not even use the displacement. 


**** maybe instead of a glass Princepled shader I would try a translucent/glossy shader and see how that looks.

*** Dont copy my settings exactly I did them real quick. Play around with the gradient top settings.

*** I probably wouldn't go with a noise texture to mix the two princepled shaders. It would be better to paint those by hand, so you can follow the curves. 

Hope this helps a little.